[In Progress] Modernize to PHP 8 WPCS 3

[gmazzap]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes/features)
• [ ] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature

What is the current behavior? (You can also link to an open issue here)

See:

• 57

• 59

• 60

• 64

• 67

In a nutshell, make compatible with PHP 8+and its features and WPCS 3

What is the new behavior (if this is a feature change)?

Dependencies are updated, compatibility improved. Still work to do to cover all modern PHP 8 things.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Yes.

Other information:

The target version/2 branch will be the branch used to release the new major.